In JP2006-269223A, an actual power generation characteristic (actual IV characteristic) of a fuel cell is estimated based on a voltage difference ΔV (=Vb−Vr) between a reference voltage Vb obtained from a reference power generation characteristic (reference IV characteristic) stored for each temperature and an actual voltage Vr. Specifically, since there is a linear relationship “ΔV=AI+B” between a deviation of the actual voltage from the reference power generation characteristic and a current at that time, A and B can be solved by giving two values to each of ΔV and I. By substituting all currents I into the above relational expression and subtracting the obtained ΔV from the reference IV characteristic, an actual IV characteristic line can be drawn.